Zigbee is usually used for outdoor sensor-nets because it’s somewhat better than Z wave at traveling through water. Rainy days can kill Z wave connections outdoors.
Regarding your mailbox, anything RF inside a metal box has trouble, wired or not. Often the best way to handle this is to create some kind of “window” that the signal can travel more easily through. Sometimes it’s just a matter of drilling a hole and then sealing it with something that is waterproof but allows the signal to pass. Some people just replace the mailbox door with a slightly smaller one and use weatherstrip around the edge and that’s enough to let the signal through.
You might also try healing the network. It’s one of those “can’t hurt, might help” things, and is considered a best practice for any time you move a mesh network device from one physical location to another.
Zigbee devices don’t simply orient to the strongest signal unless they are in table building mode. Each device is only aware of part of the total network, its “neighbors,” and it will continue to try to use its original neighbors even if moved miles away from them until you force a rebuild of the address tables.
However, if you want constant feedback, mesh is likely not the best solution. You might consider instead Wi-Fi sensors. They have much better range, constant feedback, and there’s no issues with address tables if you physically move the device . They just cost more in both money and Energy use.
I don’t know what’s available in the UK, but many people in the US use Kumostat wireless tags for long-range outdoor use cases. They can be quite nicely integrated with SmartThings as well Thanks to some work by a community member. I would imagine even if they are not available for the UK that there are other similar devices. Just another possibility to consider.